Retransmission method for multiple antenna system

ABSTRACT

A packet retransmission method for use in a communication system transmitting a signal via at least two antennas is disclosed. The packet retransmission method includes the steps of: a) receiving a signal corresponding to a specific packet, and independently transmitting a signal via the at least two antennas corresponding to the received signal; b) receiving a NACK signal associated with the packet; and c) independently applying an STTD to a real part and an imaginary part of each signal transmitted at step (a), and transmitting the STTD-resultant signal. Therefore, the method separates a real part and an imaginary part from each other, obtains a STTD gain, and can efficiently transmit a packet.

TECHNICAL FIELD

The present invention relates to a packet retransmission method for usein a communication system using a plurality of antennas, and moreparticularly to a method for constructing a new signal using a real partand an imaginary part of a transmission signal when a broadband wirelessaccess system transmits packets via a plurality of antennas.

BACKGROUND ART

FIG. 1 is a block diagram illustrating a MIMO (Multiple Input MultipleOutput) system. The MIMO system shown in FIG. 1 performs signaltransmission/reception using a plurality of antennas, such that it cantransmit desired data at high speed. Provided that the number ofantennas of a base station is set to “M”, the number of terminalantennas is set to “N”, and the number “M” is higher than the number“N”, capacity of a downlink MIMO channel is proportional to the number“N”, the base station simultaneously transmits N data streams, resultingin the implementation of high-speed data transmission.

In the meantime, an automatic repeat request (ARQ) method controls areception end to determine the presence or absence of erroneoustransmission packets, transmits the determined result to a transmissionend, and retransmits the erroneous packet when the presence of theerroneous transmission packets is determined. If the presence or absenceof the erroneous packets is fed back to the reception end, the ARQmethod transmits an acknowledgement (ACK) signal to the reception endwhen the absence of the erroneous packets is determined, and transmits anegative acknowledgement (NACK) signal to the reception end when thepresence of the erroneous packets is determined.

A Hybrid Automatic Repeat Request (HARQ) method is implemented byapplying a channel coding method to the ARQ method. In more detail,although an unexpected erroneous packet is detected from among receivedpackets, the HARQ method does not discard the erroneous packet, itcombines the erroneous packets with retransmitted packets, and decodesthe combined result, such that it increases a diversity gain and acoding gain. In the meantime, the ARQ method transmits ACK/NACK signalsusing a high-layer signaling process, resulting in the occurrence of anincreased delay. The HARQ transmits the ACK/NACK signals using aphysical-layer signaling process, resulting in a reduced delay. Whenpackets are transmitted via a plurality of antennas according to theabove-mentioned HARQ method, the above-mentioned HARQ scheme hasdifficulty in determining how to apply the space-time coding method toretransmission packets.

DISCLOSURE OF INVENTION

An object of the present invention can be achieved by transmitting afirst signal corresponding to a packet via the at least two antennasindependently, receiving a negative acknowledgement (NACK) signalcorresponding to the packet and transmitting a second signalcorresponding to the packet, applying independently Space Time TransmitDiversity (STTD) to a real part and an imaginary part of the secondsignal.

In another aspect of the present invention, provided herein is a packetretransmission method in a system transmitting a signal via threeantennas, the method comprises the step of transmitting a first, asecond, and a third signal corresponding to a packet via a first, asecond, and a third antenna, respectively, receiving step of a negativeacknowledgement (NACK) signal associated with the packet, and a step oftransmitting a retransmission signal, which is generated using a realpart of the first signal and an imaginary part of the second signal, viathe third antenna.

Preferably, the signal generated using the real part of the secondsignal and the imaginary part of the third signal may be transmitted viathe first antenna. Further, the signal generated using the real part ofthe third signal and the imaginary part of the first signal may betransmitted via the second antenna.

In other aspect of the present invention, provided herein is a packetretransmission method in a system transmitting a signal via threeantennas, the method comprises the step of transmitting a first, asecond, a third, and a fourth signal corresponding to a packet via afirst, a second, a third, and a fourth antenna, respectively, the stepof receiving a negative acknowledgement (NACK) signal associated withthe packet and the step of transmitting a retransmission signal, whichis generated using a real part of the first signal and an imaginary partof the second signal, via the third antenna or the fourth antenna.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a MIMO (Multiple Input MultipleOutput) system; and

FIG. 2 is a flow chart illustrating a packet retransmission methodaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

An initial transmission packet of a communication system capable oftransmitting a signal via three transmission antennas is transmitted viaeach antenna as denoted by the following Equation 1:

$\begin{matrix}\begin{bmatrix}s_{1} \\s_{2} \\s_{3}\end{bmatrix} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

With reference to Equation 1, S_(i) is indicative of a signaltransmitted via an i-th antenna.

In this case, a reception end receives a signal via at least threereception antennas, and the received signal can be represented by thefollowing Equation 2:

x=Hs+v  [Equation 2]

With reference to Equation 2, x is indicative of a reception signal, His indicative of a channel matrix, s is indicative of a transmissionsignal vector, and v is indicative of noise.

The above Equation 2 can also be represented by the following Equation3:

$\begin{matrix}\begin{matrix}{\left\lbrack \begin{matrix}{x_{1}(t)} \\{x_{2}(t)} \\{x_{3}(t)}\end{matrix} \right\rbrack = {{\left\lbrack \begin{matrix}h_{11} & h_{12} & h_{13} \\h_{21} & h_{22} & h_{23} \\h_{31} & h_{32} & h_{33}\end{matrix} \right\rbrack\left\lbrack \begin{matrix}s_{1} \\s_{2} \\s_{3}\end{matrix} \right\rbrack} + \left\lbrack \begin{matrix}v_{1} \\v_{2} \\v_{3}\end{matrix} \right\rbrack}} \\{= {{\left\lbrack \begin{matrix}h_{1} & h_{2} & h_{3}\end{matrix} \right\rbrack\left\lbrack \begin{matrix}s_{1} \\s_{2} \\s_{3}\end{matrix} \right\rbrack} + \left\lbrack \begin{matrix}v_{1} \\v_{2} \\v_{3}\end{matrix} \right\rbrack}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In the meantime, if a NACK signal associated with the initialtransmission packet is received, the initial transmission packet isre-transmitted. In a communication system transmitting signals via threeantennas, a method for transmitting may be represented as the followingEquation 4:

$\begin{matrix}\begin{bmatrix}{- s_{2}^{*}} \\s_{1}^{*} \\s_{3}^{*}\end{bmatrix} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

With reference to Equation 4, when a packet is re-transmitted, a firstantenna uses a conjugate of a complex signal transmitted from a secondantenna during an initial transmission time, multiplies the conjugateresult by a negative (−) sign, and transmits the multiplied result.

In the meantime, the second antenna transmits the conjugate of thesignal initially transmitted via the first antenna. And, third antennaretransmits the signal initially transmitted via the third antenna.

In this way, when the packet is retransmitted, an antenna having beenused at the initial transmission is changed among three antennas, theconjugate or the negative (−) conjugate is applied to the transmissionof the retransmitted packet, such that the reception end can obtain aSpace Time Transmit Diversity (STTD) gain.

An initial transmission packet of a communication system capable oftransmitting a signal via four transmission antennas is transmitted viaeach antenna as denoted by the following Equation 1:

$\begin{matrix}\begin{bmatrix}s_{1} \\s_{2} \\s_{3} \\s_{4}\end{bmatrix} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

With reference to Equation 1, S_(i) is indicative of a signaltransmitted via an i-th antenna.

In the meantime, if a NACK signal associated with the initialtransmission packet is transmitted, the initial transmission packet isretransmitted. A method for transmitting a signal associated with aretransmission packet via four antennas is represented by the followingEquation 6:

$\begin{matrix}\begin{bmatrix}{- s_{2}^{*}} \\s_{1}^{*} \\{- s_{4}^{*}} \\s_{3}^{*}\end{bmatrix} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

With reference to Equation 6, when a packet is re-transmitted, a firstantenna uses a conjugate of a complex signal transmitted from a secondantenna during an initial transmission time, multiplies the conjugateresult by a negative (−) sign, and transmits the multiplied result.

In the meantime, the second antenna uses a conjugate of a complex signalinitially transmitted from the first antenna, and transmits theconjugate signal. A third antenna uses a conjugate of a complex signalinitially transmitted from a fourth antenna during, multiplies theconjugate result by a negative (−) sign, and transmits the multipliedresult. The second antenna uses a conjugate of a complex signalinitially transmitted from the first antenna, and transmits theconjugate result.

In this way, when the packet is retransmitted, an antenna having beenused at the initial transmission time is changed among four antennas,the conjugate or the conjugate and negative multiplication is applied tothe retransmission of the packet, and the above-mentioned conjugatecalculation result is transmitted to a reception end, such that thereception end can obtain a Space Time Transmit Diversity (STTD) gainusing the conjugation calculation result.

If the system equipped with three antennas receives an initialtransmission packet and a retransmission packet, the received signal canbe represented by the following Equation 7:

x=As+v  [Equation 7]

With reference to Equation 7, x is indicative of an initial receptionsignal vector or a retransmission reception signal, A is indicative of achannel matrix when a signal is initially transmitted or re-transmitted,s is indicative of a transmission signal, and v is indicative of a noisevector.

The above Equation 7 can also be represented by the following Equation8:

$\begin{matrix}{\begin{bmatrix}{x_{1}(t)} \\{x_{2}(t)} \\{x_{3}(t)} \\{x_{1}^{*}\left( {t + 1} \right)} \\{x_{2}^{*}\left( {t + 1} \right)} \\{x_{3}^{*}\left( {t + 1} \right)}\end{bmatrix} = {{\begin{bmatrix}h_{11} & h_{12} & h_{13} \\h_{21} & h_{22} & h_{23} \\h_{31} & h_{32} & h_{33} \\h_{12}^{*} & {- h_{11}^{*}} & h_{13}^{*} \\h_{22}^{*} & {- h_{21}^{*}} & h_{23}^{*} \\h_{32}^{*} & {- h_{31}^{*}} & h_{33}^{*}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\s_{3}\end{bmatrix}} + \begin{bmatrix}v_{1} \\v_{2} \\v_{3} \\v_{4} \\v_{5} \\v_{6}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

In this case, in order to restore a transmission signal vector s usingthe reception signal vector x, a variety of detection techniques (i.e.,a maximum likelihood method, a MMSE (Minimum Mean Square Error) method,and a zero-forcing (ZF) method, etc.) can be used. The above-mentioneddetection techniques can also be applied to the system equipped withfour antennas in the same manner as in the above-mentioned systemequipped with three antennas.

As described above, only the first and second antennas among threeantennas have the STTD structure, and the third antenna retransmits areceived signal without any change, a diversity gain cannot be acquiredfrom a signal S₃. In the meantime, all signals can acquire a SpaceDiversity in the system equipped with four antennas, but a real part andan imaginary part of each signal use the same antenna, such that thespace diversity cannot be sufficiently acquired.

FIG. 2 is a flow chart illustrating a packet retransmission methodaccording to the present invention.

Referring to FIG. 2, in order to allow all signals S₁, S₂, and S₃ of thesystem equipped with three antennas to obtain space diversity, a realpart and an imaginary part of each retransmission packet signal areseparated, and the STTD is applied to the real part and the imaginarypart, respectively.

As denoted by Equation 8, a real part and an imaginary part of aretransmission packet signal for use in the system equipped with threeantennas are separated.

In the meantime, a real part and an imaginary part for use in the systemequipped with four antennas can be separated from each other as denotedby the following Equation 9:

$\begin{matrix}\begin{bmatrix}{{\pm {{Re}\left( s_{3} \right)}} \pm {\; {{Im}\left( s_{2} \right)}}} \\{{\pm {{Re}\left( s_{1} \right)}} \pm {\; {{Im}\left( s_{3} \right)}}} \\{{\pm {{Re}\left( s_{2} \right)}} \pm {\; {{Im}\left( s_{1} \right)}}}\end{bmatrix} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack \\\begin{bmatrix}{{\pm {{Re}\left( s_{2} \right)}} \pm {\; {{Im}\left( s_{3} \right)}}} \\{{\pm {{Re}\left( s_{1} \right)}} \pm {\; {{Im}\left( s_{4} \right)}}} \\{{\pm {{Re}\left( s_{4} \right)}} \pm {\; {{Im}\left( s_{1} \right)}}} \\{{\pm {{Re}\left( s_{3} \right)}} \pm {\; {{Im}\left( s_{2} \right)}}}\end{bmatrix} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

With reference to Equations 9 and 10, the sign of Re(s_(i)) or Im(s_(j))is determined to be a positive (+) sign or a negative (−) sign.

A reception signal association with an initial transmission packet foruse in a system capable of transmitting a signal via three antennas canbe represented by the following Equation 11:

x ₁ =H ₁ s+v ₁  [Equation 11]

The above-mentioned Equation 11 can also be represented by the followingEquation 12:

$\begin{matrix}{{\begin{bmatrix}{{Re}\left( x_{1} \right)} \\{{Im}\left( x_{1} \right)}\end{bmatrix} = {{\begin{bmatrix}{{Re}\left( H_{1} \right)} & {- {{Im}\left( H_{1} \right)}} \\{{Im}\left( H_{1} \right)} & {{Re}\left( H_{1} \right)}\end{bmatrix}\left\lbrack \begin{matrix}{{Re}(s)} \\{{Im}(s)}\end{matrix} \right\rbrack} + \begin{bmatrix}{{Re}\left( v_{1} \right)} \\{{Im}\left( v_{1} \right)}\end{bmatrix}}}\mspace{79mu} {where}\mspace{79mu} {{{{Re}(s)} = \begin{bmatrix}{{Re}\left( s_{1} \right)} \\{{Re}\left( s_{2} \right)} \\{{Re}\left( s_{3} \right)}\end{bmatrix}},{{{Im}(s)} = \begin{bmatrix}{{Im}\left( s_{1} \right)} \\{{Im}\left( s_{2} \right)} \\{{Im}\left( s_{3} \right)}\end{bmatrix}}}} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack\end{matrix}$

The reception signal association with the retransmission packet can berepresented by the following Equation 13:

$\begin{matrix}{\mspace{79mu} {{x_{2} = {{H_{2}\begin{bmatrix}{{\pm {{Re}\left( s_{3} \right)}} \pm {\; {{Im}\left( s_{2} \right)}}} \\{{\pm {{Re}\left( s_{1} \right)}} \pm {\; {{Im}\left( s_{3} \right)}}} \\{{\pm {{Re}\left( s_{2} \right)}} \pm {\; {{Im}\left( s_{1} \right)}}}\end{bmatrix}} + v_{2}}}\mspace{79mu} {x_{2} = {{{H_{3}{{Re}(s)}} + {H_{4}\; {{Im}(s)}} + {v_{2}\begin{bmatrix}{{Re}\left( x_{2} \right)} \\{{Im}\left( x_{2} \right)}\end{bmatrix}}} = {{\begin{bmatrix}{{Re}\left( H_{3} \right)} & {- {{Im}\left( H_{4} \right)}} \\{{Im}\left( H_{3} \right)} & {{Re}\left( H_{4} \right)}\end{bmatrix}\begin{bmatrix}{{Re}(s)} \\{{Im}(s)}\end{bmatrix}} + \begin{bmatrix}{{Re}\left( v_{2} \right)} \\{{Im}\left( v_{2} \right)}\end{bmatrix}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 13} \right\rbrack\end{matrix}$

With reference to Equation 13, H₃ is equal to H₂P₃, H₄ is equal to H₂P₄,and P₃ and P₄ are denoted by the following Equation 14:

$\begin{matrix}{{P_{3} = \begin{bmatrix}0 & 0 & {\pm 1} \\{\pm 1} & 0 & 0 \\0 & {\pm 1} & 0\end{bmatrix}},{P_{4} = \begin{bmatrix}0 & {\pm 1} & 0 \\0 & 0 & {\pm 1} \\{\pm 1} & 0 & 0\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack\end{matrix}$

The value

$\quad\begin{bmatrix}{{Re}(s)} \\{{Im}(s)}\end{bmatrix}$

can be calculated by Equations 13 and 14.

In the meantime, P₃ and P₄ may select one of many matrixes shown in thefollowing Equation 15 as necessary.

$\begin{matrix}{\begin{bmatrix}0 & 0 & {\pm 1} \\{\pm 1} & 0 & 0 \\0 & {\pm 1} & 0\end{bmatrix},\begin{bmatrix}0 & {\pm 1} & 0 \\0 & 0 & {\pm 1} \\{\pm 1} & 0 & 0\end{bmatrix},\begin{bmatrix}{\pm 1} & 0 & 0 \\0 & 0 & {\pm 1} \\0 & {\pm 1} & 0\end{bmatrix},\begin{bmatrix}0 & 0 & {\pm 1} \\0 & {\pm 1} & 0 \\{\pm 1} & 0 & 0\end{bmatrix},\begin{bmatrix}0 & {\pm 1} & 0 \\{\pm 1} & 0 & 0 \\0 & 0 & {\pm 1}\end{bmatrix},} & \left\lbrack {{Equation}\mspace{14mu} 15} \right\rbrack\end{matrix}$

Although the system includes at least four reception antennas, it candetect a transmission signal using the following Equation 16:

$\begin{matrix}{\mspace{79mu} {{x_{2} = {{H_{2}\begin{bmatrix}{{\pm {{Re}\left( s_{2} \right)}} \pm {\; {{Im}\left( s_{3} \right)}}} \\{{\pm {{Re}\left( s_{1} \right)}} \pm {\; {{Im}\left( s_{4} \right)}}} \\{{\pm {{Re}\left( s_{4} \right)}} \pm {\; {{Im}\left( s_{1} \right)}}} \\{{\pm {{Re}\left( s_{3} \right)}} \pm {\; {{Im}\left( s_{2} \right)}}}\end{bmatrix}} + v_{2}}}\mspace{79mu} {x_{2} = {{{H_{3}{{Re}(s)}} + {H_{4}\; {{Im}(s)}} + {v_{2}\begin{bmatrix}{{Re}\left( x_{2} \right)} \\{{Im}\left( x_{2} \right)}\end{bmatrix}}} = {{\begin{bmatrix}{{Re}\left( H_{3} \right)} & {- {{Im}\left( H_{4} \right)}} \\{{Im}\left( H_{3} \right)} & {{Re}\left( H_{4} \right)}\end{bmatrix}\begin{bmatrix}{{Re}(s)} \\{{Im}(s)}\end{bmatrix}} + \begin{bmatrix}{{Re}\left( v_{2} \right)} \\{{Im}\left( v_{2} \right)}\end{bmatrix}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 16} \right\rbrack\end{matrix}$

With reference to Equation 16, H₃ is equal to H₂P₃, H₄ is equal to H₂P₄,and P₃ and P₄ are denoted by the following Equation 17:

$\begin{matrix}{{P_{3} = \begin{bmatrix}0 & {\pm 1} & 0 & 0 \\{\pm 1} & 0 & 0 & 0 \\0 & 0 & 0 & {\pm 1} \\0 & 0 & {\pm 1} & 0\end{bmatrix}},{P_{4} = \begin{bmatrix}0 & 0 & {\pm 1} & 0 \\0 & 0 & 0 & {\pm 1} \\{\pm 1} & 0 & 0 & 0 \\0 & {\pm 1} & 0 & 0\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 17} \right\rbrack\end{matrix}$

In the meantime, P₃ and P₄ may select one of many matrixes shown in thefollowing Equation 18 as necessary.

$\begin{matrix}{\begin{bmatrix}0 & {\pm 1} & 0 & 0 \\{\pm 1} & 0 & 0 & 0 \\0 & 0 & 0 & {\pm 1} \\0 & 0 & {\pm 1} & 0\end{bmatrix},\begin{bmatrix}0 & 0 & {\pm 1} & 0 \\0 & 0 & 0 & {\pm 1} \\{\pm 1} & 0 & 0 & 0 \\0 & {\pm 1} & 0 & 0\end{bmatrix},\begin{bmatrix}0 & 0 & 0 & {\pm 1} \\0 & 0 & {\pm 1} & 0 \\0 & {\pm 1} & 0 & 0 \\{\pm 1} & 0 & 0 & 0\end{bmatrix},\begin{bmatrix}0 & {\pm 1} & 0 & 0 \\{\pm 1} & 0 & 0 & 0 \\0 & 0 & {\pm 1} & 0 \\0 & 0 & 0 & {\pm 1}\end{bmatrix},\begin{bmatrix}0 & 0 & {\pm 1} & 0 \\0 & 0 & 0 & {\pm 1} \\0 & {\pm 1} & 0 & 0 \\{\pm 1} & 0 & 0 & 0\end{bmatrix},\begin{bmatrix}0 & 0 & 0 & {\pm 1} \\0 & 0 & {\pm 1} & 0 \\{\pm 1} & 0 & 0 & 0 \\0 & {\pm 1} & 0 & 0\end{bmatrix},\begin{bmatrix}{\pm 1} & 0 & 0 & 0 \\0 & {\pm 1} & 0 & 0 \\0 & 0 & 0 & {\pm 1} \\0 & 0 & {\pm 1} & 0\end{bmatrix},\begin{bmatrix}0 & 0 & 0 & {\pm 1} \\0 & 0 & {\pm 1} & 0 \\{\pm 1} & 0 & 0 & 0 \\0 & {\pm 1} & 0 & 0\end{bmatrix},\begin{bmatrix}0 & 0 & {\pm 1} & 0 \\0 & 0 & 0 & {\pm 1} \\0 & {\pm 1} & 0 & 0 \\{\pm 1} & 0 & 0 & 0\end{bmatrix},} & \left\lbrack {{Equation}\mspace{14mu} 18} \right\rbrack\end{matrix}$

According to the present invention, the above-mentioned method issuperior to a Forward Error Correction (FEC)-based retransmissiontechnique in light of a transmission power aspect for acquiring the sameerror probability.

The above-mentioned method according to the present invention has a highperformance superior to that of a conventional STTD structure. Theconventional STTD structure is more efficiently operated in a low-speedenvironment than in a high-speed environment, but the above-mentionedmethod according to the present invention can be very sufficientlyapplied to both the high-speed environment and the low-speedenvironment.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

INDUSTRIAL APPLICABILITY

The present invention may apply to a mobile communication system.

1. A packet retransmission method in a communication system transmittinga signal via at least two antennas, the method comprising the steps of:a) transmitting a first signal corresponding to a packet via the atleast two antennas independently; b) receiving a negativeacknowledgement (NACK) signal corresponding to the packet; and c)transmitting a second signal corresponding to the packet, applyingindependently Space Time Transmit Diversity (STTD) to a real part and animaginary part of the second signal.
 2. The method of claim 1, whereinthe step c) includes the steps of: c1) generating the second signalusing the real part and the imaginary part of the first signal; and c2)selecting antennas excluding the antennas associated with the real partand the imaginary part of the first signal; and c3) transmitting thesecond signal via the selected antennas.
 3. A packet retransmissionmethod for use in a system capable of transmitting a signal via threeantenna, the method comprising the steps of: a) transmitting a first, asecond, and a third signal corresponding to a packet via a first, asecond, and a third antenna, respectively; b) receiving a negativeacknowledgement (NACK) signal associated with the packet; and c)transmitting a retransmission signal, which is generated using a realpart of the first signal and an imaginary part of the second signal, viathe third antenna.
 4. The method of claim 3, further comprising the stepof: transmitting a second retransmission signal, which is generatedusing a real part of the second signal and an imaginary part of thethird signal, via the first antenna.
 5. The method of claim 4, furthercomprising the step of: transmitting a third retransmission signal,which is generated using a real part of the third signal and animaginary part of the first signal, via the second antenna.
 6. A packetretransmission method in a system transmitting a signal via fourantennas, the method comprising the steps of: a) transmitting a first, asecond, a third, and a fourth signal corresponding to a packet via afirst, a second, a third, and a fourth antenna, respectively; b)receiving a negative acknowledgement (NACK) signal associated with thepacket; and c) transmitting a retransmission signal, which is generatedusing a real part of the first signal and an imaginary part of thesecond signal, via the third antenna or the fourth antenna.
 7. Themethod of claim 6, further comprising the step of: d) transmitting a newsignal, which is generated using a real part of the third signal and animaginary part of the fourth signal, via the first antenna or the secondantenna.